The present invention relates to the extraction of mineral values from mineral-containing materials. In a more specific aspect, the present invention relates to the extraction of mineral values in situ from subsurface formations. In a still more specific aspect, the present invention relates to the extraction of uranium values in situ from subsurface formations containing uranium.
Numerous minerals are present in subsurface earth formations in very small quantities which make their recovery extremely difficult. However, in most instances, these minerals are also extremely valuable, thereby justifying efforts to recover the same. An example of one such mineral is uranium. However, numerous other valuable minerals, such as copper, nickel, molybdenum, rhenium, silver, selenium, vanadium, thorium, gold, rare earth metals, etc., are also present in small quantities in subsurface formations, alone and quite often associated with uranium. Consequently, the recovery of such minerals is fraught with essentially the same problems as the recovery of uranium and, in general, the same techniques for recovering uranium can also be utilized to recover such other mineral values, whether associated with uranium or occurring alone. Therefore, a discussion of the recovery of uranium will be appropriate for all such minerals.
Uranium occurs in a wide variety of subterranean strata such as granites and granitic deposits, pegmatites and pegmatite dikes and veins, and sedimentary strata such as sandstones, unconsolidated sands, limestones, etc. However, very few subterranean deposits have a high concentration of uranium. For example, most uranium-containing deposits contain from about 0.01 to 1 weight percent uranium, expressed as U.sub.3 O.sub.8 as is conventional practice in the art. Few ores contain more than about 1 percent uranium and deposits containing below about 0.1 percent uranium are considered so poor as to be currently uneconomical to recover unless other mineral values, such as vanadium, gold and the like, can be simultaneously recovered.
There are several known techniques for extracting uranium values from uranium-containing materials. One common technique is roasting of the ore, usually in the presence of a combustion supporting gas, such as air or oxygen, and recovering the uranium from the resultant ash. However, the present invention is directed to the extraction of uranium values by the utilization of aqueous leaching solutions. There are two common leaching techniques for recovering uranium values, which depend primarily upon the accessibility and size of the subterranean deposit. To the extent that the deposit containing the uranium is accessible by conventional mining means and is of sufficient size to economically justify conventional mining, the ore is mined, ground to increase the contact area between the uranium values in the ore and the leach solution, usually less than about 14 mesh but in some cases, such as limestones, to nominally less than 325 mesh, and contacted with an aqueous leach solution for a time sufficient to obtain maximum extraction of the uranium values. On the other hand, where the uranium-containing deposit is inaccessible or is too small to justify conventional mining, the aqueous leach solution is injected into the subsurface formation through at least one injection well penetrating the deposit, maintained in contact with the uranium-containing deposit for a time sufficient to extract the uranium values and the leach solution containing the uranium, usually referred to as a "pregnant" solution, is produced through at least one production well penetrating the deposit. The present invention is directed to the latter, "in situ" leaching.
The most common aqueous leach solutions are either aqueous acidic solutions, such as sulfuric acid solutions, or aqueous alkaline solutions, such as sodium carbonate and/or bicarbonate.
Aqueous acidic solutions are normally quite effective in the extraction of uranium values. However, aqueous acidic solutions generally cannot be utilized to extract uranium values from ore or in situ from deposits containing high concentrations of acid-consuming gangue, such as limestone. Aqueous alkaline leach solutions are applicable to all types of uranium-containing materials and are less expensive than acids.
The uranium values are conventionally recovered from acidic leach solutions by techniques will known in the mining art, such as direct precipitation, selective ion exchange, liquid extraction, etc. Similarly, pregnant alkaline leach solutions may be treated to recover the uranium values by contact with ion exchange resins, precipitation, as by adding sodium hydroxide to increase the pH of the solution to about 12, etc.
As described to this point, the extraction of uranium values is dependent to some extent upon the economics of mining versus in situ extraction and the relative costs of acidic leach solutions versus alkaline leach solutions. However, this is an oversimplification, to the extent that only uranium in its hexavalent state can be extracted in either acidic or alkaline leach solutions. While some uranium in its hexavalent state is present in ores and subterranean deposits, the vast majority of the uranium is present in its valence states lower than the hexavalent state. For example, uranium minerals are generally present in the form of uraninite, a natural oxide of uranium in a variety of forms such as UO.sub.2, UO.sub.3, UO.U.sub.2 O.sub.3 and mixed U.sub.3 O.sub.8 (UO.sub.2.2UO.sub.3), the most prevalent variety of which is pitch blende containing about 55 to 75 percent of uranium as UO.sub.2 and up to about 30 percent uranium as UO.sub.3. Other forms in which uranium minerals are found include coffinite, carnotite, a hydrated vanadate of uranium and potassium having the formula K.sub.2 (UO.sub.2).sub.2 (VO.sub.4).sub.2.3H.sub.2 O, and uranites which are mineral phosphates of uranium with copper or calcium, for example, uranite lime having the general formula CaO.2UO.sub.3.P.sub.2 O.sub.5.8H.sub.2 O. Consequently, in order to extract uranium values from subsurface deposits with aqueous acidic or aqueous alkaline leach solutions, it is necessary to oxidize the lower valence states of uranium to the soluble, hexavalent state.
Combinations of acids and oxidants which have been suggested by the prior art include nitric acid, hydrochloric acid or sulfuric acid, particularly sulfuric acid, in combination with air, oxygen, sodium chlorate, potassium permanganate, hydrogen peroxide and magnesium dioxide, as oxidants. Alkaline leachants and oxidants heretofore suggested include carbonates and/or bicarbonates of ammonium, sodium or potassium in combination with air, oxygen or hydrogen peroxide, as lixiviants. However, sodium bicarbonate and/or carbonate have been used almost exclusively in actual practice.
While the previous discussion would indicate that "in situ" recovery of mineral values, such as uranium, is fairly simple and straight forward and would appear to be the best technique in most cases, except for the volumes of leach solution required, the very nature of subsurface formations containing mineral values and the types of formations in which such mineral values are found seriously complicate "in situ" recovery.
The general practice is, of course, to complete both the injection wells and the production wells through the entire vertical dimension of the formation of interest. Accordingly, the body of leach solution travels in a radial pattern and in a horizontal direction from the injection well or wells to the producing well or wells. It is known from experience, in the leaching of mineral values as well as the injection of drive fluids in secondary and tertiary recovery of oil, that the area of the reservoir actually contacted by the injection fluids is relatively small, simply because the injected fluids do not flow in a uniform radial pattern and it is wholly impractical to drill a sufficient number of injection and production wells to take full advantage of the natural flow patterns. This lack of adequate "aereal" sweep or contact of the formation is further seriously complicated by the fact that the porosity of a formation is seldom uniform and the injected fluids will have a tendency to follow fissures, high permeability streaks, etc. in traveling from the injection wells to the producing wells. Therefore, improvement of the aereal sweep of the formation is highly desirable.
In addition to the above, the mineralized formation may sometimes be bounded by porous formations above or below the formation of interest and in many cases, such zones are of higher porosity than the zone of interest and are of greater vertical dimensions. Accordingly, substantial volumes of the injected fluid are lost in these thief zones. Therefore, it is also highly desirable to reduce this loss of injected fluids.
At the present time, all commercial operations for the recovery of mineral values, particularly uranium, are believed to be confined to "wet" formations which are located below the water table and which have a natural water drive which augments the flow of injected fluids through the formation. However, there are a number of uranium containing deposits in "dry" formations which lack a natural water drive, usually those located above the water table. In many cases, even though these formations are relatively shallow, they cannot be practically or economically recovered by conventional mining means and "in situ" recovery is the only alternative. However, there are presently no known techniques available for the recovery of uranium from these formations which lack a natural water drive.